This invention relates to a self-sustaining solenoid in which a plunger is retracted by a supply of an operating current and held at its retracted position even after cutting off of the operating current and moved back upon application of a return current.
In conventional types of self-sustaining solenoids, when an operating current is applied to a coil, the plunger is attracted into the coil to butt against a permanent magnet fixed to one inner end of the coil and, even after cutting off of the operating current, the plunger is retained on the permanent magnet by its magnetic attractive force. By applying to the coil a current reverse in direction from the operating current to apply to the permanent magnet a magnetic field opposite to the direction of magnetization of the permanent magnet the attractive force of the permanent magnet is reduced and the plunger is pulled out of the coil to its original position, for example, by the force of a return spring imparted to the plunger beforehand.
Since the prior art self-sustaining solenoid has such a construction that during operation the plunger makes direct contact with the permanent magnet as described above, the permanent magnet is liable to be broken. Further, an oxide magnet has been used, as the permanent magnet, which has a relatively large coercive force and a Curie temperature above 450.degree. C. and is difficult of demagnetization. The magnetization of such a permanent magnet is not affected by the application thereto of the magnetic field based on the operating current or return current. Accordingly, when the plunger stays in its restored state, it is likely to be attracted by the permanent magnet if moved a little by mechanical vibration or shock towards the permanent magnet. If the force of the return spring is made large to avoid the possibility of such accidental attraction of the plunger by the permanent magnet, it is necessary to increase the number of turns of a coil for attracting the plunger against the return spring during operation, and further, means for holding the plunger at its restored position inevitably becomes bulky, resulting in the overall size of the device being increased.
It is an object of the present invention to provide a self-sustaining solenoid which is long-lived even if used frequently.
Another object of the present invention is to provide a self-sustaining solenoid which can be constructed small.
Another object of the present invention is to provide a self-sustaining solenoid which stably maintains its restored states even if subjected to mechanical vibration or shock and which can be constructed small.
Yet another object of the present invention is to provide a self-sustaining solenoid which is hardly affected by an ambient temperature change or power source voltage fluctuation.